The present invention relates to the general field of non-destructive inspection of a transparent ceramic thermal barrier layer formed on a substrate.
A particular field of application of the invention is that of adhesion testing for coatings such as those used for hot parts in a turbine, such as for nozzle vanes or for rotor blades in a high pressure turbine.
It is known to provide parts made of monocrystalline metal alloy with a thermal barrier in the form of a ceramic layer. A ceramic that is commonly used for this purpose is zirconia ZrO2, possibly stabilized with yttrium. Such a thermal barrier is typically formed by physical vapor deposition (PVD), and in particular by electron beam PVD (EBPVD), which gives a columnar structure to the thermal barrier.
A bonding underlayer is interposed between the substrate of monocrystalline metal alloy and the thermal barrier, which has a function of withstanding oxidation, and which enhances adhesion of the thermal barrier so as to enable it to present good resistance to spalling.
A known bonding underlayer is made up of a nickel and/or cobalt aluminide that has been modified with a platinum type metal, and that may possibly also include, in the vicinity of the interface with the thermal barrier, a metal that promotes the formation of a film of alumina (Al2O3) on which the thermal barrier is anchored.
For monocrystalline metal alloy parts provided with a thermal barrier formed by a ceramic oxide layer, and more generally for substrates coated with a ceramic layer, it is desirable to have available a method that makes it possible in non-destructive manner to determine whether any losses of cohesion have appeared between the oxide layer and the substrate.
Document FR 2 926 137 discloses a method of determining the adhesion of a ceramic coating layer, which method consists in applying a laser pulse to the substrate coated in the ceramic layer, in searching for an image representative of a zone of separation, if any, between the ceramic layer and the substrate, and in analyzing the detected image, if any. In the absence of separation of the ceramic layer, and since this layer is transparent, it leaves visible the underlayer, which is gray in color. In contrast, in the event of the ceramic layer being separated as a result of the laser pulse, a layer of air is present under the separated portion of the ceramic layer, which gives rise to a spot that is pale. The area of the spot represents the magnitude of the separated portion.
In general, it is essential to be able to measure accurately the spots observed at the interface between the ceramic coating layer and the substrate in order to be able to estimate the sizes of the corresponding separations. Unfortunately, given the poor contrast that exists between the color of the spot (generally whitish) and the gray color of the underlying layer, those spots are often difficult to observe and define with the naked eye or while using any simple optical technique.